[brian64]

Given that he didn't cap pre-flop, we can guess he doesn't have a great hand. In fact, he could be one of the guys who raises most of the time with the button, so he could have anything. When you raise pre-flop, he has to guess you have some kind of good two-way hand with A2 or A3. The flop comes with two low cards, and he still raises you, so he probably has at least a decent high. I think we have to credit him with at least 2-pair, in which case your equity won't be much above 50%, so no reason to raise. But even if he has a set, you still have decent equity, and raising couldn't be that big of a mistake because he might just be a maniac. After the turn, it is looking grim for the high, but just the low draw makes it worth a call getting 5-1. Even if the low doesn't come on the river, I would probably still check/call getting 6-1.